Proposed New Radio (NR) radio access technologies are similar to Long Term Evolution (LTE) technologies based on orthogonal frequency division multiplexing (OFDM). Transmissions in LTE (and many other wireless communication schemes) are organized in subframes. In case of LTE, a subframe consists of 14 consecutive OFDM symbols. The subframe structure provides a common time reference between the transmitter and receiver as well as a structured way of describing the relation between, for example, data and the associated control signaling. The transmission of messages start at the beginning of a subframe (or a fixed time related to the start of a subframe).
Uplink (UL) and downlink (DL) messages are typically scheduled. In other words, for each time instant the scheduler determines to which device to transmit messages and which device(s) that are supposed to transmit messages. These scheduling decisions are communicated to the devices using Layer 1/Layer 2 (L1/L2) control signaling. In LTE, the L1/L2 control signals are located at the beginning of each one millisecond (ms) subframe. The device is, for each subframe, monitoring for control signaling relevant for this device. A number of candidate messages are monitored using so called blind decoding where the device attempts to decode a number of candidate control messages in each subframe and determine whether any of these messages are for the device in question. The larger the number of control channel candidates the larger the flexibility in the network operation. For instance, in terms of supporting different features potentially requiring differently-sized control messages, but also the higher the device complexity in terms of blind decoding attempts.
For data transmission, hybrid automatic repeat request (HARQ) functionality is often employed where the receiver can request retransmission of erroneous data from the transmitter. To support this functionality there is a need for feedback from the receiver to the transmitter (or scheduler) in the form of a HARQ acknowledgement. From a latency point-of-view, the HARQ acknowledgements should come as soon as possible after data reception.
Defining the transmissions of data and/or messages in terms of subframes results in several drawbacks. For example, the transmissions are restricted to start at subframe boundaries only. The subframe duration thus has a direct impact on the lowest possible latency. Moreover, operation in unlicensed spectrum typically requires a listen-before-talk procedure. Once the radio channel is declared available, data transmissions should ideally start as soon as possible to avoid other user equipment grabbing the channel. Restricting transmissions to start at subframe boundaries only will thus either result in decreased likelihood of grabbing the channel or, if a dummy signal is transmitted until the start of a subframe, unnecessary overhead. Furthermore, to enable relaxed synchronization requirements, it is advantageous to enable later data transmissions in a subframe and by that create a guard period at the beginning of the subframe. The restrictions inherent in the use of subframes for data transmissions do not permit such flexibility.